Massive timber elements in roofs moisture performance

Size: px

Start display at page:

Download "Massive timber elements in roofs moisture performance"

Aubrey Simpson
6 years ago
Views:

1 Massive timber elements in roofs moisture performance Berit Time, Research Manager, Dr.ing SINTEF Building and Infrastructure Stig Geving, Senior Researcher, Dr.ing SINTEF Building and Infrastructure Knut Magnar Sandland, Research Manager, Dr.scient. Norsk Treteknisk Institutt (NTI) KEYWORDS: roof, massive timber elements, moisture transport, vapour barrier SUMMARY: This paper presents an evaluation of the hygrothermal performance of massive timber roof constructions. An ongoing project Massive timber properties and use is aiming towards giving recommendations for how to build sustainable massive timber roofs. Activities like water vapour permeability measurements of wood, hygrothermal simulations, field measurements and following up several ongoing building projects is being done in the project in order to fulfil the aim 1. Introduction In Norway there are long traditions for using wood in building constructions. Most houses in Norway except for apartment buildings are for instance lightweight timber frame houses and an increasing amount of smaller apartment buildings are built in wood. In Norway the Building Regulations, which are based on European Union s Construction Product Directive, are performance based and has been so since This means that there are opportunities to build multi-storey buildings in wood as long as the requirements in the Construction Product Directive (Council Directive 89/106/EEC) on building and civil engineering works are judged to be fulfilled. The timber industry is important for Norway and forest raw materials represent a considerable renewable resource, with a large unexploited potential for a wide range of applications. The Norwegian government, through different research and development programmes, support initiatives that leads to development in the use of timber. The wood sector of Norway has as it s most important aim to increase the use of wood in order to increase the growth of value in the wood industry and in the forest sector. Wooden buildings have become more popular lately also because of its environmental feature and sustainability. Massive wood constructions are made of massive wood elements. These elements are in principal multi-layer cross-laminated timber panels (fig. 3). The panels can be tied together mechanically or by use of glue. Massive wood constructions are more often used in central Europe and in Austria in particular. In Norway there are a couple of factories producing these kinds of elements and the building sector also imports elements from the neighbouring countries. The manufacturers report about high activity and a huge interest in this building technique.

2. Modern architecture and roof performance 2.

There is a great focus on renewing/developing the wood based architecture in Norway and many bigger cities name themselves wooden cities (e.g Bergen, Stavanger, Trondheim and Elverum).

1, are examples of modern high-lighted projects. The architecture of these new projects are claimed to be innovative and creative, future-oriented, environmentally friendly and universal Fig.

2 2. Modern architecture and roof performance 2.1 New architectural trends Many Norwegian architects these days see a great potential in applications for massive timber in an urban context and for environmental friendly design in general. There is a great focus on renewing/developing the wood based architecture in Norway and many bigger cities name themselves wooden cities (e.g Bergen, Stavanger, Trondheim and Elverum). The timber-built-village at Siriskjær in Stavanger (approximately m 2 of dwellings), fig. 2 and the Preikestolen Mountain Lodge (high standard lodge with 29 bedrooms), fig. 1, are examples of modern high-lighted projects. The architecture of these new projects are claimed to be innovative and creative, future-oriented, environmentally friendly and universal Fig. 1 Sketch of the Preikestolen Moutain Lodge ( Architect: Helen & Hart Fig. 2 Illustration of the timber-built-village at Siriskjær in Stavanger (Architect: Studio Ludo + A.A.R.T) Fig. 3 Examples of mounting of massive timber elements, roof on walls. Fig. 4 It is recommended to raise massive timber constructions using Weather Protection Systems Fig. 5 Modern dwellings built of massive timber element Fig. 6 Part of a new school being built in massive timber

3 As have been experienced in for instance Sweden (Vessby 2007) Norwegian architects and engineers have a great focus on keeping the massive timber visually exposed to at least parts of the indoor surroundings. Their reason for this is often aesthetical and/or related to a wish of profiting on the woods ability to be a moisture buffer (see e.g Hameurey 2006) and also related to environmental and indoor reasons. Buildings built in massive timber can be competitive compared to e.g concrete slabs and steel roofing elements, and in addition there are several advantages (renewable raw material, possibilities for solutions of high flexibility, low weight) and builders/craftsmen report that it is very easy to work with. When using massive timber elements in floors and walls there are however, some challenges, e.g. sound transmission in the construction and exposure for bad weather in the building period. Roofs made of massive timber elements can be regarded having a great potential for use, and therefore it is of interest to increase the knowledge to attain optimal building solutions for this purpose. 2.2 Sharing of knowledge It has been claimed/questioned among practicioners and experienced by SINTEF that there is a need for more knowledge in terms of climate adaption, constructive timber detailing and building physics for these kinds of constructions. New building regulations that imply mm of thermal insulation have also risen a question about moisture performance. In a development project running these days on Massive timber properties and use one of the project goals is to study and make guidelines and recommendations on how to build sustainable and well performing roofs by use of massive timber. SINTEF Building and Infrastructure give to the Norwegian building sector on planning and building through the Building Design Sheet. Advices and recommendations given in the Building Design Sheets are typically well documented recommendations. It is a goal for the project to implement recommendations for this kind of roof constructions in a Building Design Sheet (In Norwegian : Byggforskserien). 2.3 Type of roofs In Norway roofs are being built according to one of two main principles in order to meet overall performance criteria mentioned Compact roofs (warm roofs) Ventilated roofs (cold roofs) Compact roofs are roofs with no airspace between the material layers and shall as a main rule according to SINTEF recommendations have water downlets inside the building (Norwegian climate zones). The roofs can be either flat or sloped and they must be covered with a roofing material that can resist water pressure. Ventilated roofs are roofs that need to be ventilated beneath the roof coverings so that the roof should be kept cold, snow should not melt and the excess of moisture should be ventilated. The roof water downlets should be outside the building envelope and the roof should preferably have a slope of at least degrees (Building Design Sheet ). Massive timber roofs can be built according to both these principles though the most common way of doing it up to now has been to make compact flat roofs. 2.4 Challenges and often raised questions Challenges and questions often asked among architects and engineers for massive timber roofs in relation to building physics are connected to Wooden compact roofs and moisture issues/-problems Should there be a vapour barrier in this kind of roofs? Temperature levels around the downlets/-pipes in compact roofs and minimum requirements for thermal insulation thickness by roof drain (pipes)

4 The air tightness of massive timber element roofs Moisture protection during the construction process 3. Activities in the project In order to be able to give good answers to the questions raised it was decided to work with the following activities in the ongoing project: Measurement of water vapour permeability through wood and glued wood at different relative humidity levels Hygrothermal simulations of relevant massive timber roof constructions Measurements of hygrothermal properties in a compact flat roof in an apartment building Cooperation and communication with contractors and assessments of constructions being built Development of recommendations and general drawings for massive timber roof detailing 3.1 Water vapour permeability of wood with and without glue It is a well known fact that water vapour transport in wood is dependent on moisture level and temperature. Literature show great variations in water vapour permeability in wood (Time 1998). The water vapour permeability is often said to be approximately 4 times higher for wood in equilibrium with 100 % relative humidity than for dry wood. Since the internal variations in wood properties vary considerably, more consistent values on vapour permeability for same quality of wood (used as massive timber elements) with and without glue depending on relative humidity levels should be available. A modification of the cup test method has been done in order to measure water vapour properties for wood and glued wood at different relative humidity and hysteresis levels. The test samples have been made so that same samples in a lid can easily be moved from one cup (i.e one specific RH saltsolution) to another. For further description of the test method, see NS-EN ISO (2001). The measurements are in process and the results from these measurements will be presented at the symposium. 3.2 Hygrothermal simulations of relevant massive timber constructions In wooden architecture and engineering there is and has always been a certain focus on the necessity of using a vapour barrier in massive timber constructions. This is the case for walls as well as for roofs. Roofs built with massive timber can be compact roofs or ventilated roofs. The main function of a vapour barrier is to maintain the airtightness of the building element and to reduce the transport of water vapour from the interior. Moisture transported through air leakages normally represents a greater risk for moisture problems and building damages than vapour transport by diffusion. Both these mechanisms have to be considered when assessing and recommending built up of roofs. Initial hygrothermal calculations of compact roof elements have been performed and more calculations are in process. The main objective of the present work is to document how different roof solutions (i.e mainly with a vapour barrier, without a vapour barrier or with a vapour retarder) in relation to thickness of thermal insulation and wooden timber element type of materials in the construction built-in-moisture (level) for the massive timber element exterior climate indoor air humidity level. Based on previous experience and hygrothermal simulations, the project aims to present a practical tool for assessing the necessity of using a vapour barrier for different kinds of roof constructions, for different built in moisture levels and for different climate zones.

5 3.2.1 Initial calculations Initial calculations have been performed for a compact roof. For description of the roof construction and the different cases simulated see table 1. Table 1 The roof construction is consisting of (seen from the cold side/outdoor climate side) for 4 different calculation cases: Case One-layer bitumen roofing x x x x 250 mm mineralwool x x 80 mm mineralwool x x 0,15 mm PE-foil (vapour barrier) x x 150 mm massive timber element x x x x The purpose of the initial calculations was to consider the need for a vapour barrier in a specific roof being built in Trondheim. The building was one of the buildings studied in this project and the contractors were worried about the hygrothermal performance in relation to the area of the roof around the roof drain were the thickness of the thermal insulation is at a minimum. In that particular area the thermal resistance of the massive timber element might be at the same level or even higher than the thermal insulation. With a vapour barrier on top of the timber element the RH below the vapour barrier might get to high, and giving an increased risk for mould growth.the calculated cases with an insulation thickness of 80 mm should simulate the situation by the drainage down lets. The calculations have been performed with WUFI 1D Pro 4. The transport mechanisms considered in the calculation tool are water vapour diffusion and capillary transport. The tool includes the moisture capacity of the material layers. Air leakages are not considered. The climatic data used are the so called Moisture Design Reference Year (MDRY) for Trondheim. The calculation period has been selected to 5 years.the indoor air humidity level have been defined by the moisture supply, which is the difference between indoor and outdoor air water vapour content. The moisture supply was chosen to 4 g/m 3 for outdoor temperatures below 0 C, and decreasing linearly to 1 g/m 3 at an outdoor temperature of 20 C. The built in moisture level has been set in equilibrium with 80 % RH. This is considered to be fairly conservative as we experience that the contractors have become more aware of moisture problems in the building process and precautions are taken especially in relation to wood and wooden elements. For the particular project a WPS-system (tent) were used during construction. Material parameters from WUFI material database have been used. A µ-value for the bitumen roofing membrane of 600 has been used and 108 (at 0 % RH) and decreasing linearly to 27 (at 100 % RH) has been used for wood. The glue layers in the massive timber elements have not been accounted for since our measurements are still not reported. Some results for these initial calculations are given in the two example diagrams in figure 7 and Discussion The calculations show (fig.7) that the cases without a vapour barrier obtain the lowest RH in the outer parts of the constructon (i.e in the mineralwool just below the membrane) during the summer months. The results show that for the cases with a vapour barrier the moisture transport will be stopped before it reaches/is absorbed by the massive timber elements. It is to be seen that the inward drying is particularly large especially for case 3 (with 80 mm of insulation and no vapour barrier) and that it starts early in the spring. For case 1 (with 250 mm of insulation and no vapour barrier) the calculations show that the drying process starts later than for case 2 and 4 (with vapour barriers). Fig. 8 shows that the RH in the outer parts of the timber elements (close to the mineralwool and the vapour barrier) is very much dependent on material layers (vapour barrier/no vapour barrier) and the insulation thickness. For the cases with 80 mm thickness of the mineralwool it is seen that the RH in the outer parts of the wood is lower for the case with no vapour barrier. This is because the thermal resistance of the wooden layer is approximately on the same level as the thermal resistance of the insulation layer and the vapour barrier is placed in a position more than (the recommended by SINTEF) 25 % inside the thermal resistance of the roof. However it is to be considered that for both cases the RH and temperature levels will be below the critical limit for mould growth (i.e below 80 % RH while the outdoor temperature is below 0 C).

6 100 RH in the outer parts of the mineralwool (just below the roofing) 80 No.4 RH (%) No.1 60 No. 2 No. 1 No. 2 No. 3 No.3 No Time (hours) Fig.7 RH in the outer mm of the mineralwool (just below the roofing membrane). The 5 years calculation period starts on the 1 of January the first year. RH in the outer mm of the wood 100 RH (%) No.1 80 No.4 60 No.2 No.3 40 No No. 2 No. 3 0 No Time (hours) Fig. 8 RH in the outer parts of the massive timber elements (close to the mineralwool and the vapour barrier). The 5 years calculation period starts on the 1 of January the first year. For the cases with 250 mm of mineralwool we can see a greater risk for mould growth for the construction without a vapour barrier likely due to summer condensation and also because a greater amount of built in moisture is transported outwards compared to the case with 80 mm thickness of thermal insulation. The inward vapour transport during the summer will be stopped by the vapour barrier if it is there, if not the moisture will be absorbed by the massive timber element and give an increased RH. If it is considered that mould growth on the wooden parts near the insulation is the potential risk of this construction, it is judged, according to these initial calculations, to be safe to use a vapour barrier between the mineralwool and the timber element. This material layer/the barrier can also serve as a contribution to the airtightness of the roof. The calculations also show that in the areas around the roof drain the need for a vapour barrier is less important if the airtightness can be dealt with. Anyway for the assumptions/considerations used in these calculations the use of a vapour barrier should not be any problem.

7 The airtightness of massive timber element constructions are very much dependent on how the separate elements are connected to each other. The authors are not aware of any documentation of the airtightness of massive timber elements. Further calculations will be done in order to be able to give more detailed advices related to indoor moisture level, initial moisture level (e.g for built under-roof constructions), other insulation thicknesses, different climate zones and for recommended minimum levels for insulation thicknesses by the water down lets. 3.3 Field measurements of hygrothermal properties in a roof Norsk Treteknisk Institutt (NTI) has set up moisture measurements in two separate massive timber roofs in a newly built apartment building in Hokksund in the southern part of Norway. The built up of the roof can be seen in fig. 9. There is a vapour barrier in one roof and no vapour barrier in the other. Apart from that they are equal. It has been focus on reducing the weather exposure of the massive timber elements during construction by the aid of not permanent covers. RH levels have been recorded for approximately one year and concerning differences in moisture levels within the two roofs, only minor difference can be seen at this stage between the roof, having and not having a vapour barrier (fig. 10 and fig.11). A small difference can be seen in the reduction of the RH level just below the roofing throughout the winter and spring. The measurements will carry on some more years in order to record first, second and third year results. (ill.: Toverød 2006) Fig. 9 A sketch showing the roof in which RH levels are recorded. It is an almost flat roof (6 ), 12 meter wide, with a ventilated roof construction. The roofing is 3 mm PVC-membrane on a 16 mm OSB-board, 100 mm ventilated air gap, wind barrier, 270 mm mineralwool, vapour barrier alt. no vapour barrier, 80 mm massive timber elements. Fig. 10 RH level measurements at 4 different positions in a roof without a vapour barrier above the massive timber element

8 Fig. 11 RH level measurements at 4 different positions in a roof with a vapour barrier above the massive timber element 4. Concluding remarks The paper presents some building physic challenges experienced in connection with massive timber roof constructions. An ongoing project Massiv timber properties and use is aiming towards giving recommendations for how to build massive timber roofs. Activities like water vapour transport measurements, hygrothermal simulations, field measurements and following up several ongoing building projects are being done in the project in order to fulfil the aim. 5. Acknowledgements The content of this paper is a part of the ongoing project Massive timber properties and use financed by the Research Council of Norway, The Forest Owner Association, Moelven Massivtre AS, Holz100 Norge AS, Dynea ASA, Moelven Limtre AS, Heimdal Gruppen and Norsk Treteknisk Institutt, The paper has also been supported by the project Climate Adapted Buildings (CAB) funded by The Research Council of Norway. The authors gratefully acknowledge the supporters of the project. 6. References Byggdetaljer (2007) Valg av taktype og konstruksjonsprinsipp, Byggforskserien, SINTEF Byggforsk, Oslo 2007 (In Norwegian). Vessby (2007), Sveriges högsta moderna bostadshus i trä: Limnologen i Växjö, Bygg & teknikk 2/07 (In Swedish) Time, B., (1998) Hygroscopic Moisture Transport in Wood, Doktor Ingeniør thesis, Norwegian Univ. of Science and Technology, Dept. of Building and Construction Engineering, February Hameurey S. (2006), The Hygrothermal Inertia of Massive Timber Constructions, Dr. Thesis, KTH, Sweden. NS-EN ISO (2001). Hygrothermal performance of building materials and products -- Determination of water vapour transmission properties, CEN European Committee for Standardization.

Moisture and temperature conditions in cold lofts and risk of mould growth

Moisture and temperature conditions in cold lofts and risk of mould growth Sivert Uvsløkk, M. Sc. Norges byggforskningsinstitutt; sivert.uvslokk@byggforsk.no http://www.byggforsk.no/ KEYWORDS: roof, loft,